It's a bit of a leap to assume that behavior like that is evidence of nefarious activity; ask yourself, if you were implementing some sort of secret signalling layer, would you design one that uses simple English words, and whose failure state reveals its presence?
Consider the other cases - it could be something like an anti-spam system; back when MSN Messenger was a thing, it would abruptly close any conversations in which certain virus-related keywords were said (mostly including ".exe", which is how I discovered it), presumably in an attempt to stop them spreading.
Or it could be a bug - there have been plenty of these, from simple strings causing mass IRC disconnections, to eerie conspiracy theories (see "bush hid the facts"[0], a conspiracy caused by a bug in Notepad).
Or it could just be a bit of debug code accidentally being triggered. This past week, I had to explain to some users why a website was talking about "DEAD BEEF". The reason was innocent (glitch in a web server config), but to the end user it was incomprehensible.
All that said, if you want to investigate further, it's simple to disable iMessage on iOS devices. If you go to Settings->Messages, there's a toggle for it.
[0]: https://en.wikipedia.org/wiki/Bush_hid_the_facts